A novel n-type semiconducting biomaterial

There has been no research conducted thus far on the semiconducting behaviour of biomaterials. In this study, we present an n-type semiconducting biomaterial composed of amorphous kenaf cellulose fibre (AKCF) paper with a voltage-controlled N-type negative resistance. The AKCF generates an alternating-current wave with a frequency of 40.6 MHz from a direct-current voltage source at its threshold voltage (electric field of 5.26 kV/m), which is accompanied by a switching effect with a four-order resistance change at 293 K. This effect is attributed to the voltage-induced occurrence of strong field domains (electric double layers) at the cathode and depletion at the anode of the AKCF device. The proposed AKCF material presents considerable potential for applications in flexible/paper electronic devices such as high frequency power sources and switching effect devices.


S1. Methods
Dried bast kenaf pulp fibres (harvested in Bangladesh, Toho Tokushu Pulp Corp. Kitakami, Japan) were retted in a 2.6 % water solution for 18 ks. They were then defibrated in a mixer for 1.2 s. Subsequently, they were miniaturized through ball milling using zirconia balls for 36 ks. The sample structure was analysed through X-ray diffraction (XRD) in the reflection mode with monochromatic Cu Kα radiation. A selected-area electron diffraction (SAED) analysis was performed using transmission electron microscopy (JEM-2100, JEOL). The surface morphologies were analysed via atomic force microscopy (NanoScope V/Dimension Icon, Bruker AXS). All electronic measurements were performed in an Al shield box to present the results from being affected by the electromagnetic interference from surroundings. Fig. S1 presents a schematic diagram of the experimental circuit.

S2. TEM image of AKCF
The cellulose structure is characterised by a mixed structure, which primarily comprises irregular lined-up nanofibrils along with a small quantity of non-fibrous fibrils. The cellulose bundles are tied up with nanofibrils with a diameter of 0.36 nm to form a cellulose nanofiber (CNF) with a diameter of approximately 4 nm.   Profiles 1 (b) and 2 (c) depict the heights measured from the valley along the black lines in the AFM image (a), revealing convex distances of 10.6 and 27.0 nm, respectively. The diameters are three to five times larger than that of AKCF, which is ~4 nm, as shown in Fig. 3b. This could be attributed to the condensation of cellulose bundles due to strong thixotropy of the AKCF. Thixotropy is an effect in which the viscosity depends on both the velocity gradient as well as the time for which force has been applied 28 . The faster a thixotropic liquid moves, the less viscus it becomes.

S5. The calculated energy band structure of AKCF
The molecular structure of cellulose of both kenaf and softwood are identical.
Consequently, the density of states for the C12H20O10 molecule optimized from the local structure of cellulose was simulated. Figure S5 depicts a band gap of -4.77 eV 8,9 .

S7. Correlation between dielectric response and DC conductivity
Tokura et al. 29 reported a significant experimental correlation between dielectric response such as dielectric relaxation and DC conductivity for various organic ionic donoracceptor charge-transfer compounds. They observed through an experimental analysis that kink-type domain walls exist between one-dimensional ferroelectric molecular domains. In this study, we observed ideal dielectric relaxation (Fig. 3d) and large DC conductivity (Fig. 1d)    considered as an electron avalanche, since the AKCF is not a diode that comprises a p-n junction but a passive semiconductor device with two terminals comprising only n-type semiconductor material.

S8. Depth dependence of frequency
The frequencies were determined by samples of varying thickness, not by an external circuit. We observed a correlation between the frequency and depth of specimens (Fig.   S7c) based on the following values: 60.4 MHz at 19 μm (Fig. 2c), 9.4 MHz at 28 μm (Fig.   S6a), and 9 kHz at 74 μm (Fig. S7b). Figure S7b depicts the sixth-order harmonic AC waves with frequencies of 1 kHz, 2.4 kHz, 3.8 kHz, 5.2 kHz, 6.6 kHz, and 9 kHz.
However, the reason behind the generation of these harmonics remains unclear. We observed two semicircles in the Nyquist diagram presented in Fig. 3c. It can be observed from Fig. S8 that the electric resistivities obtained from the small and large semicircles belong to the semiconductor group.

S9. Transition of electrons in bands
A Gunn diode is characterised by electron transfer between a lower band and a higher band 22-24 , as shown in Fig. S9. Therefore, we must consider the electron mass and drift velocity in the upper band. Firstly, we consider the electron mass. Since the electric dipole domain is formed by an electron and proton pair in a CNF, the drift velocity of the pair is determined by the heavier mass (1.67 × 10 -24 g) of the proton. The mass ratio of m2 in the upper band to m1 in the lower band is 1836: 14 (1.67 × 10 -24 + 9.11× 10 -28 ) g /9.11× 10 -28 g), such that m2 = 1836.14 m1.
Gunn diode is based on the transport of a strong electric field region (domain), and its period of modulated current depends on the travel time of dipole domains. Thus, the ratio of m2/m1 and R2C2 constant of the upper band conveniently serve as a measure of electric capability and the time constant in place of the effective mass and drift time, respectively.